The Pioneer LX-91 will go brilliantly with KRP-500 I have seen the LX-91 and it is superlative quality and CD play back is superb, Build quality are first rate, and looks are excellent, you can buy with confidence and not have to worry about upgrading anytime time soon, i saw this with a 5090 and blew me away, i did not think a Blu-ray player could be any better than what i have seen already, i thought the FD-51 was good especailly for the money, but the LX-91 is in a different class, the Denon 3800 is a great player and was very impressed, but again the pioneer edged it some way, the picture are super sharp, and colours you feel like you could dive in and swim in them, i am biased towards pioneer products but this is genuinly a player to beat, i should imagine the only player that will be as good, if not better is the new all singing Denon which comes out soon this has SACD play back and is a monster BUT this will set you back £3,500-£4000 and i can say that i can not see the denon being "£2000 better thn this player, i thought this about the FD-51 and asked myself if this player LX-91 was worth the extra £1200 or so more, but it truly is.
If you can try to get to have a dem, even it means travelling a bit it will be worth it then at least you can decide for your self if it is better than the Marrantz, or it could be that you like the Marrantz more, either way both players would give you much enjoyment, but my money would be on the pioneer.
Just for your info as well WHFSAV will be carrying out a group test with high end Blu-ray players including Pioneer,Denon, Sony and the Marrantz so it might be worth hanging on untill you read the out come of the test, but at the end of the day it can be only you that can decide which player you like, so do try and get a dem.
Hope this helps a bit.
Andy